Holographic Media Research Articles

Passive introduction of carrier fringes in real-time photorefractive interferometers for single interferogram analysis

We propose a new technique to introduce carrier fringes in an holographic interferometric set-up for deformation or vibration analyses. This technique makes use of the anisotropic diffraction processes of photorefractive crystals. The main advantage of the proposed technique is that it just involves static components, no moving parts or active components are required. This technique is demonstrated in a real-time interferometric set-up operating at the wavelength of 1.06 µm and with a GaAs photorefractive crystal as the holographic medium.

Code retrieval via undercover multiplexing

The purpose of this research is to develop an undercover multiplexing technique to give additional protection for optical information encryption. We employ the double random phase mask as our basic optical encryption system. The holographic storage medium of choice is a photorefractive crystal. To achieve the multiplexing we use the aperture size of the pupil in the optical system, as it governs the speckle size. We introduce such variation in order to produce a decorrelation between two consecutively stored speckle patterns. Each stored speckle pattern is associated to an input encrypted image, thus producing a multiplexing of the encrypted information. We implement this operation without altering the setup architecture and the random phase masks. This multiplexing is our undercover operation to encipher a true code behind a fake code. Under this approach, the user can only recover the bulk information stored in the volume hologram. However, he cannot recover the true code without the additional information on the pupil size key, even if accessed in position of the original decoding mask.

The Theory of Light Diffraction in Thin Anisotropic Medium

The coupled wave theory of Raman and Nath diffraction is extended to the case of thin anisotropic holographic media with grating vector parallel to the medium boundaries. Solutions for the wave amplitudes, diffraction efficiencies, and angular mismatch sensitivities are given in transmission geometries for the case of dielectric modulation. The main difference of the new results with respect to the expressions valid for isotropic media arises due to the walk-off between the wave-front and energy propagation directions. The difference is particularly important in materials with large birefringence, such as organic crystals, ordered polymers, and liquid crystalline cells.

Evaluation of polycarbonate substrate hologram recording medium regarding implication of birefringence and thermal expansion

In this paper, we evaluate photopolymer media using a polycarbonate (PC) substrate. In holographic data storage medium, substrates that sandwich the photopolymer material are needed to protect the photopolymer material against exogenous shock and open air. An optical glass such as BK-7 is normally used as a substrate, but a PC substrate has a cost advantage and is easy to fabricate compared with optical glass. For holographic recording and reading, however, the high birefringence and high thermal expansion of a PC substrate are significant problems. First, we analyze the degree of degradation of output power by the polarization change and estimate the threshold value of birefringence to record hologram normally. Next, we estimate the temperature tolerance of hologram readout with polycarbonate substrate hologram medium. These analyses results indicate the possible usage of the PC substrate as holographic recording media.

Characterization of thin real-time holographic media by dithered four-wave mixing

Photosensitive optical media of the types that are used in real-time holography generally exhibit both photorefractive and photochromic response. The parameters that characterize each of these types of response can be extracted by detailed quantitative analysis of four-wave mixing experiments in the Raman–Nath regime. This principle has been tested using reactive eyeglass lenses. The techniques of Fourier optics have been applied to the analysis of diffraction from amplitude/phase gratings produced in the photochromic glass by laser illumination. Dithering of a laser-induced fringe pattern produces a combination of two-wave and four-wave mixing that can be analysed in terms of amplitudes proportional to products of Bessel functions of different orders. The incident and diffracted beams are predicted, and observed, to be modulated at the dither frequency and its harmonics. The small dither amplitudes that were employed, made possible by use of a phase-sensitive detection technique, allowed Bessel function expansions to be truncated after only a few terms for the purpose of parameter estimation. The technique has allowed the very small refractive index change associated with dispersion of the photo-induced absorption in silver halide doped photochromic glass to be measured for the first time.

Photopolymerizable recording media for three-dimensional holographic optical memory

The development of investigations in recent years on photopolymerizable recording holographic media for using as photosensitive layers of holographic optical disks, which satisfy the conditions for the application in archive optical memory is analyzed. It is shown that the main emphasis in the development of photopolymerizable holographic recording media is put on the improvement of their component composition. Formulations that permit joint radical and cationic photopolymerization were designed for thermally stable holographic recording. The holographic characteristics achieved for these media ensure the fabrication of single-layer optical disks 150 mm in diameter with the information capacity of 200 GB instead of 5 GB for modern optical disks. Ways of improving photopolymerizable holographic recording media for the manufacture of optical disks with an information capacity of more than 1 Tb cm−3 are considered.

Improvement in Temperature Tolerance of Holographic Data Storage Using Wavelength Tunable Laser

In this paper, we report a new method of increasing the temperature tolerance of holographic recording media using a wavelength tunable laser. Of all holographic media, photopolymer media are particularly sensitive to temperature, thus some methods are required to maintain a low bit-error ratio (BER) as temperature changes. We demonstrate here how to maintain a high signal-to-noise ratio (SNR) as temperature changes using a wavelength tunable blue external cavity laser diode (ECLD) currently under development. For comparison, we analyze the temperature tolerance by a angle-tuning method and a wavelength-tuning method. We have focused that a hybrid of wavelength-tuning and angle-tuning method, we can read out the full of the image at 60°C, which is +30°C above the recording temperature. Moreover, we propose a recording method that allows the wavelength to be adjusted to the temperature changes. By adjusting the wavelength linearly to the temperature changes during recording, we can read out the holograms recorded at different temperatures using the wavelength corresponding to the readout temperature.

Optical data storage in LC cells

AbstractLiquid crystal devices as a medium for holograms storage have been investigated. Long term memory effects in LC cells have been observed. Experiments proved that certain combination of insulating alignment layers has a major influence on the long term memory effect. Optimal liquid crystal cell construction allows us to achieve sufficient diffraction efficiency to record holographic patterns and to develop a re-writable holographic medium. The configuration of PVK and polyimide layers in LC cell construction with specific LC mixture was tested. The method of permanent and re-writable recording of optical data (holographic pattern) onto LC cells was achieved. However, the method of erasing recorded data was realized but mechanisms of this phenomenon are not clearly understood yet.

Holographic Fourier synthesis of dynamic Moiré-like patterns in the photorefractive <inline-formula><math altimg="none" display="inline" overflow="scroll"><mrow><msub><mi>Bi</mi><mn>12</mn></msub><msub><mi>TiO</mi><mn>20</mn></msub></mrow></math></inline-formula> crystal

We propose and physically discuss a kind of holographic Fourier synthesis in the profile of dynamic Moiré-like patterns. The interference patterns are obtained by progressively controlling the superposition of more than two dynamic sinusoidal phase gratings. It represents a new holographic optical nondestructive testing routine in real time for metrological purposes. The sinusoidal phase gratings with slightly different pitches are dynamically generated by photorefraction in the Bi12TiO20 (BTO) crystal sample. The photorefractive holographic medium is used in anisotropic self-diffraction configuration and in a diffusion-only recording mechanism.

Dynamic holographic endoscopy?ex vivo investigations of malignant tumors in the human stomach

Laser holographic interferometry is based on the superimposition of the holograms of different motional states of an object on a single holographic storing medium. Using a combination of holographic interferometry and endoscopic imaging, we tried to detect areas of focally disturbed tissue elasticity in gastric cancer preparations. By connecting a mobile electronic speckle pattern interferometry (ESPI) camera system (light source: double frequency Nd:YAG laser, lambda = 532 nm) to different types of endoscopes, ex vivo experiments were performed on ten formalin fixed human stomachs, nine containing adenocarcinomas and one with a gastric lymphoma. Linking the endoscopic ESPI camera complex to a fast image processing system, the method of double pulse exposure image subtraction was applied at a video frame rate of 12.5 Hz. Speckle correlation patterns and corresponding phase difference distributions resulting from gastric wall deformation by gentle touch with a guide wire were analyzed. Tumor-free gastric areas showed high-contrast concentric fringes around the point of stimulation. In contrast, fringe patterns and filtered phase difference distributions corresponding to the areas of malignancy in all the cases were characterized by largely parallel lines, indicating that stimulation of rigid tumor tissue primarily led to tilting. Our ex vivo investigations of malignant gastric tumors show that the application of dynamic holographic endoscopy makes it possible to distinguish areas of malignancy from surrounding healthy tissue based on the differences in tissue elasticity.

Theoretical description of the photorefractive detection of the ultrasound modulated photons in scattering media

Acousto-optic imaging of thick biological tissues can be obtained in real-time with an adaptive-wavefront holographic setup, where the holographic media is a self-developping photorefractive crystal. As a consequence, the interference signal resulting from the acousto-optic effect can be easily collected with a high etendue and fast single photodetector. We present a statistical model of the field propagating through the scattering media and show why the various acoustic frequency components contained in the speckle output pattern are uncorrelated. We then give a detailed description of the signal measured through the photorefractive effect, in order to explain the quadratic pressure response observed for the two commonly used configurations setup e.g an amplitude or a phase modulation of the ultrasound.

Preparation and Characterization of Organic-Inorganic Nanocomposite Films for Holographic Recording

The photopolymerization of aromatic methacrylate in organic-inorganic nanocomposite films for holographic recording media was investigated. Thick photopolymer films (thickness>200 µm) were prepared using organic-inorganic hybrid solutions containing high refractive index monomers, through the sol-gel process. These photopolymer films were polymerized upon exposure to a visible light with high photo conversions. The photopolymerization was highly effective under visible light irradiation and could be applied to a holographic medium with high diffraction efficiency (>95 %) by using a 532nm laser. The diffraction efficiency of the film was much affected by morphology, which might affect monomer diffusion during the recording.

UV-Patterned Polysilane Films as a Phase Mask in Optical Correlation System

Entirely transparent polysilane films, which were modified by ultraviolet (UV) light exposure to decrease their refractive index, were used as phase masks. The phase modulation amount of the transmitted laser, which was estimated using a Mach–Zehnder interferometer, can be modified linearly by UV exposure dose, indicating that the polysilane films have a very useful property of controllability by UV light exposure. Optical correlation using the polysilane phase masks as the information-carrying medium was then demonstrated. A matched-filter four-wave-mixing optical correlator using a photorefractive (PR) polymer as a real-time holographic data storage medium was used. A large correlation signal difference of over 3.6-fold was obtained for the striped polysilane phase masks that were arranged orthogonally or in parallel. This is due to the distinct encoded images in the polysilane phasemasks originating from the large refractive index modification in polysilane films of approximately Δn=-0.14 at λ=633 nm. The high transparency in the entire visible region and such large refractive index modification make polysilane films difficult to forge, and allow us to use phase masks in pattern recognition systems for security verification.

Enhanced detection of acousto-photonic imaging signals using a photorefractive crystal based system

Acousto-photonic imaging is a dual-wave sensing technique where diffuse coherent light wave interacts with a superimposed acoustic field. A phase-modulated light wave emanates from the interaction region and carries with it information about the local opto-mechanical properties of the insonated media. The coherent nature of the light produces a speckle field. The modulation of the speckle field is spatially incoherent, yielding a small modulation depth when collecting multiple speckles or extremely low light levels when detecting at the single speckle level. We report preliminary results from a new detection scheme where the scattered laser light is mixed with a reference beam in a photorefractive crystal. The crystal serves as a dynamic holographic medium and a photorefractive grating is formed from which the reference beam diffracts. The diffracted reference beam has the same spatial structure as the scattered light, and the two interfere at the photodetector where the phase modulation on the scattered beam is converted to an intensity modulation. Measurements of the signals are presented for gel phantoms seeded with suspended polystyrene beads. The results exhibit qualitative agreement with a simple theoretical model. [Work supported by the Center for Subsurface Sensing and Imaging Systems via NSF ERC Award No. EEC-9986821.]

Holographic scheme for independent component analysis

An optoelectronic analog feedback circuit uses dynamic holography to perform independent component analysis on an array of input signals imposed on a set of optical carriers. The requisite nonlinearity for achieving high-order statistical analysis is established with optical phase modulation in the feedback loop. The dynamics of the system is shown to be driven by the statistical characteristic functions of the input signals and reveals winner-takes-all dynamics when at least one of the signals is sub-Gaussian. Signal correlation and circuit adaptation is carried out by the dynamic holographic medium and therefore takes place in a parallel fashion. The allowed signal bandwidth is limited by the delay time of the feedback loop, thus signal bandwidths exceeding 100 MHz should be possible using relatively standard electronic and optoelectronic technology. Moreover, the scheme can directly accommodate microwave carrier frequencies to several tens of gigahertz.

Holographic simultaneous readout polarization multiplexing based on photoinduced anisotropy in bacteriorhodopsin.

Bacteriorhodopsin (bR) is a reversible photochromic protein that can be used as a holographic medium. The dichroic absorption of the bR molecule is polarization dependent, thereby allowing for the recording of polarization holograms. The properties of polarization holograms can be used to multiplex two independent images in a single bR film. A new technique and associated polarization-multiplexing scheme are demonstrated that allow for simultaneous readout of two orthogonally polarized images while achieving a high normalized diffraction efficiency for each of the individual images.

Storage capacity of holo-interferograms.

The practical storage capacity of a holographic medium can be found by finely comparing reconstructions from independent holograms of an information-dense object wave. With the help of two orthogonally polarized reference waves, a pair of volume holograms is recorded simultaneously at imprint densities as high as 4.1 x 10(10) bits/cm3. As a consequence of polarization, the holograms are not mutually coherent, and the twin encodings of the object wave can be reconstructed separately. These are brought into fine registration interferometrically and then scanned by a CCD camera. Experiments on glass-mounted Agfa 8E56, a fine-grained silver halide emulsion designed for holography, are reported. When the object wave was moderately dense in information, grain noise was the main cause of the reconstruction errors. Emulsional plasticity was the more significant factor both when the object wave was optically sparse and when it was extremely dense. Plasticity noise limited the information that could be retrieved to 2.7 x 10(10) bits/cm3, which is 2 orders of magnitude below the capacity suggested by the emulsion's bandwidth and grain-noise figures alone.

HOLOMETER: measurement apparatus for the evaluation of chalcogenide glasses as holographic recording media

A unique instrument, capable of concurrently recording and evaluating various types of multiplexed holograms, is under development. Details about the hardware specifications of this so-called HOLOMETER are provided. Theoretical considerations on the specific criteria that the holographic media needs to satisfy, as well as an analysis of the diffraction efficiency of thick holographic diffraction gratings, are also presented. Finally, some preliminary results of the recording and evaluation of chalcogenide films, which have recently emerged again as potentially promising holographic data-storage media, are reported.

Code-multiplexed optical scanner.

A three-dimensional (3-D) optical-scanning technique is proposed based on spatial optical phase code activation on an input beam. This code-multiplexed optical scanner (C-MOS) relies on holographically stored 3-D beam-forming information. Proof-of-concept C-MOS experimental results by use of a photorefractive crystal as a holographic medium generates eight beams representing a basic 3-D voxel element generated via a binary-code matrix of the Hadamard type. The experiment demonstrates the C-MOS features of no moving parts, beam-forming flexibility, and large centimeter-size apertures. A novel application of the C-MOS as an optical security lock is highlighted.

Spectral-hologram-based correlator for photonic IP router: architectural considerations

A novel method for implementing header address recognition by using spectral-hologram-based correlator is proposed as a cost-effective way of realizing routing in internet protocol over photonic networks. Due to a difference of spectral response between commercially available holographic media and optical fibers, a second-harmonic optical header-address signal is generated and used both in the synthesis of the spectral hologram and in the recognition of the header addresses. The spectral hologram is employed to store the header addresses. The header-address recognition is performed by taking correlations between an incoming second-harmonic optical address and those stored in the spectral hologram. The system performance is analyzed.